ITV1 advertising revenue is forecast to fall about 12% year on year in September, dashing hopes that the worst of the advertising slump is over.

September is a critical month for determining the mood of advertisers rebounding from the quiet summer period, and analysts expected a modest single-digit decline at ITV1, which is considered a bellwether for the health of the commercial TV industry.

"This is horrible, a worrying sign that we didn't want that perhaps the issue is structural and not just cyclical due to the recession," said one TV industry source. "What was wanted was a bounce back to something like down 5%."

ITV as a whole, including ad spend across its digital channels, is expected to be down about 11% in September, media industry sources predict.

Revenue at ITV1, which accounts for 39% of all UK TV ad spend, fell 20% year on year in the first six months.

In a preview note out today ahead of ITV's interim results on 6 August, Numis predicted ITV would be down just 6% overall from September until the end of the year.

Media industry sources predict that the total UK television market will fall at least 12% in September, although some sources believe ad spend could slip down 15%, compared with a 19% year-on-year fall in September 2008.

Channel 4's flagship channel is expected to be down about 16% and the broadcaster about 13% when its total portfolio of digital channels is included.

Channel Five, which has had the toughest time among the main commercial broadcasters, is expected to be down 20% in September. A 19% fall is forecast for Five's total portfolio of channels.

BSkyB is expected to be down about 10%.

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